The invention is generally concerned with the automatic control of injection molding machines. More specifically, this invention relates to a control for an injection molding machine in which a work-indicative parameter is integrated with respect to time during the injection of plasticized material into a mold thus defining a work index which is used to control the molding machine.
In the past it has been common to employ monitoring systems and controls in injection molding apparatus. Typically, a monitor comprises apparatus for recording specified variables during each cycle of the injection molding apparatus. Such monitoring systems, however, require manual changes to be made by an operator to obtain consistently uniform products. Ordinarily, the operator reviews the recorded variables for consecutive molding cycles and then makes an appropriate adjustment to the injection molding apparatus such that acceptable products are likely to be obtained.
Control systems comprise another type of automatic control for injection molding machines. Typically, control systems sense one or more parameters which are considered to be necessarily associated with the injection molding of uniform, high quality products. Each of the sensed parameters is compared with a specified range within which the parameter must lie in order to give acceptable products. If a sensed parameter lies outside of its allowable range, a feedback system is ordinarily provided to make a compensating adjustment to an operating parameter of the injection molding machine.
With monitoring systems and their reliance on human operators, there is a substantial likelihood that a large number of non-uniform, or low quality products may be molded before the injection molding apparatus can be brought within acceptable limits. It is noted in passing that unacceptable products generate excessive manufacturing costs and waste costly plastic materials.
Control systems, by comparison, substantially reduce the number of unacceptable products by automatically making compensatory adjustments when necessary. As noted above, control systems typically sense particular parameters and maintain those parameters within specified limits. While some such controls are useful, there has persisted a need for a truly practical one capable of maintaining product quality even in the face of variations in feed stock and ambient conditions. Feed stock of synthetic resinous material varies from one batch to another and frequently contains varying proportions of virgin and reground material. Both of those characteristics may adversely affect product quality. Ambient temperature is still another variable which can affect the operation of injection molding apparatus and product quality. The above are but a few of the variables which can affect the molded parts and the minimum range which may be specified for the sensed parameters.
Aside from the variables which affect the minimum range for the sensed parameters, there remains the problem of which parameter or parameters are to be sensed as being the most representative of a quality product. A myriad of such parameters have been proposed for use heretofore, including many of the operating parameters of the injection molding apparatus such as injection ram velocity, melt temperature, melt pressure, hydraulic injection pressure, injection time interval, mold pressure, etc.
Some molding process controls have employed sensing devices to indicate when a mold cavity is filled to a specified pressure. Such sensing devices have been used to shift molding apparatus from an injection portion to a holding portion of the molding cycle. In addition such sensing devices have been used as an indication of product quality.
It has also been proposed to indicate product quality by an integral of work-parameter integrated with respect to time between positions at the beginning and the end of a ram stroke. Typically, the selected positions have been indicated by using a linear position potentiometer. This integral has been fraught with difficulty, however, since the end or bottom of a ram stroke is variable from cycle to cycle. Accordingly, a physically fixed position for the end of the stroke may allow significant product quality variations between consecutive mold cycles.
The use of mold pressure sensors alone has, likewise, been problematic. More specifically, there may be a sink region in the mold or an excessively packed region. In addition, other parameters of the molding apparatus may manifest quality-affecting excursions which are not reflected in the selection of mold pressure.
It is therefore apparent that these previously used parameters have not been altogether satisfactory for controlling injection molding apparatus.